1. Technical Field
The present invention is directed toward rotating electric machines having a slip ring.
2. Description of the Related Art
A conventional alternator 10 is illustrated in FIG. 1 sometimes referred to herein as a generator. Alternator 10 has a rotor assembly generally designated by the reference numeral 20 and stator assembly generally designated by the reference numeral 15. The rotor assembly 20 includes a shaft 21 supporting all rotating magnetic circuit structures thereof including conventional pole-members 16A and 16B, rotor core 17 and field coil 18 wound upon bobbin 12. Additionally, all other non-magnetic circuit rotating structures are carried thereby, including air circulation fans 19 and 27 located at axially opposite sides of the pole-members, and a slip ring assembly 30 located at one extreme end of the shaft. Fan 27 is formed from sheet metal stock and spot welded to pole-member 16B while fan 19 is formed from an appropriate thermoplastic material and heat staked to tower extensions (not shown) from the field coil bobbin 12. The shaft 21 in turn is rotatably supported within a housing 26 by a pair of bearings 23 and 22. Bearing 23 is located between the slip ring assembly 30 and the fan 19.
Coil leads 18A of field coil 18 are wrapped about respective posts 12A of bobbin 12 and pass through holes 13 in fan 19. Slip ring assembly 30 is made of a pair of copper rings 31, each having a slip ring lead 32 joined such as by welding thereto. The copper rings and wires are molded into a thermoset material to complete the slip ring assembly. Slip ring assembly 30 is pressed onto the end of rotor shaft 21 and the slip ring leads 32 are routed into channels along the shaft 21 where they are joined, such as by twisting and welding, to the coil leads 18A of field coil 18 via a joint 24. The joint 24 is then bent to the surface of the fan 19 and received in a pyramid-shaped tab structure 25. The joint 24 is then secured to fan 19 by ultrasonic welding of the plastic material of the tab 25. Bearing 23 is assembled to pass over the slip ring assembly 30 to retain the lead wires 32 securely within the shaft channels. The configuration in FIG. 1, however, presents several manufacturing challenges which may affect long-term durability.
First, the weld connection at joint 24 may be made imperfectly, for example, where the weld bead contains a partial crack or fissure. Such a joint will normally pass electrical tests conducted during manufacture. However, during the service life of generator 10, the rotational forces (i.e., centrifugal forces) that come to bear on joint 24, either directly or indirectly (e.g., via flexure of the fan body when it rotates) may cause the crack to propagate until the electrical connection is broken entirely, resulting in an open condition in the field winding circuit. This will cause the generator to fail.
Second, the above-mentioned ultrasonic welding operation may incompletely melt the plastic in the vicinity of the joint 24, resulting in gaps or voids. These gaps or voids may allow the wires or joint 24 itself to move during operation of the generator (i.e., rotation of the rotor). This movement may fatigue the metal, causing it to break, resulting in an open circuit and failure of the generator. Additionally, a weld horn that is used in the ultrasonic welding operation may contact the wires or joint 24 directly (not just the tab 25). The high vibration imparted by the weld horn may fatigue the wires or joint 24, perhaps not severe enough to cause a break that could be detected during manufacture via electrical testing. Moreover, the weld horn may crush, flatten or otherwise deform the wires or joint, thereby weakening it. The wires/joint, thus weakened, may fail during operation of the generator.
Third, slip ring conductor 32 may be pulled too tight when joint 24 is made, causing it to rise slightly out of the channel in the shaft and fan hub where it is designed to reside. Bearing 23, when assembled onto shaft 21, may contact conductor 32, deforming or possibly cutting the wire""s outer insulation. Such a condition will ground the rotor winding, causing the generator to fail.
Fourth, the connection of conductors 32 to respective slip rings 31 is conventionally made via brazing. As shown in FIG. 1, such connections are disposed proximate a radially outermost portion of the slip ring assembly 30. Rotational forces increase with increases in distance from the main axis, and may therefore weaken such connections, causing some to fail.
U.S. Pat. No. 5,625,244 to Bradfield discloses a slip ring and fan assembly having channels on an inside (i.e., rotor segment facing) surface of the fan for routing coil leads to the slip ring assembly. However, the channels do not provide any mechanism for retaining the leads.
There is therefore a need for an improved alternator and/or portions thereof that minimize or eliminate one or more of the problems as set forth above.
An object of the present invention is to provide a solution to one or more of the problems set forth in the Background. An advantage of a fan in accordance with the present invention is that it provides a mechanism, in cooperation with a rotor segment, to secure or retain a rotor coil lead without any additional manufacturing steps. Another advantage is that it eliminates the routing of leads on the outer, front surface of the fan (as described in the Background), eliminating the need for the front welding connection employed on conventional configurations. The present invention further minimizes or eliminates axial movement of the fan which has previously fatigued broken leads. Further, routing of the leads under the fan protects the leads from environmental conditions. Still another advantage of a fan according to the invention relates to its improved rigidity, which minimizes flexure and oscillation of the fan during operation (i.e., during the operation of an alternator that includes the inventive fan).
A fan is provided for use in a dynamoelectric machine of the type having a rotor with a rotatable shaft disposed along a longitudinal axis and a field coil having a pair of coil leads. The fan includes a hub portion having a central aperture configured to accommodate the shaft, and a carrier portion. The carrier portion extends radially outwardly from the hub and has a plurality of vanes on a first side thereof. The carrier also includes a second side opposite the first side configured to contact an engagement surface of the rotor. The carrier further includes a pair of channels on the second side for routing of the coil leads. According to the invention, when the fan is assembled into the dynamoelectric machine against the rotor, each channel has a substantially uniform depth taken relative to the rotor""s engagement surface over a predetermined radial length. In a preferred embodiment, the uniform depth is selected so as to allow the use of a compressible insulating sleeve for surrounding each coil lead. The lead/sleeve combination is compressed when the fan is assembled against the rotor. The compression provides a retention function, holding the leads in place during the remainder of the manufacture of the machine as well as during operation.